In this specification unless the contrary is expressly stated, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not to be construed as an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of common general knowledge; or known to be, relevant to an attempt to solve any problem with which this specification is concerned.
The general forms of most metal detectors that interrogate soils are one of handheld battery-operated units, conveyor-mounted units, or vehicle-mounted units. Examples of hand-held products include detectors used to locate gold, explosive land mines or ordnance, coins and treasure. Examples of conveyor-mounted units include fine gold detectors in ore mining operations, and an example of a vehicle-mounted unit includes a unit to search for land mines.
These metal detectors usually consists of a at least one or more inductors which may transmit and receive an alternating magnetic field, such inductors often referred to as “coils,” transmit electronics which may generate a transmit signal applied to at least one inductor to produce a transmitted magnetic field, at least one inductor, a first inductor, receives a magnetic field to produce a receive emf signal, the said first inductor is connected to receive electronics which may amplify and filter the receive emf signal to produce a receive signal, the receive electronics contains signal processing which may process the receive signal to produce and output indicator signal.
Magnetic soils produce relatively large interfering signals compared to typical targets sought, for example gold or landmines buried relatively deeply. To detect such targets in magnetic soils, it is necessary for the transmitted magnetic field to contain multi-frequencies and the receive signal to be processed in such a way that linear combinations of the multi-frequencies cancel out the signals from the magnetic soils whilst not canceling target signals.
Most metal detectors transmit a repeating sequence. Examples of multi-frequency metal detectors with the ability to substantially reduce magnetic soil signals are disclosed in:                1) U.S. Pat. Nos. 5,576,624, 6,636,044, 6,653,838; a pulse induction transmit waveform “time domain” detector. Receive signal relatively high frequency resistive components are most strongly manifest following immediately after the very short high back emf. This period following the back emf is a receive period, a first period, during which the transmitted magnetic field is zero. Mostly only relatively low frequency resistive components are manifest in the receive signal near the end of the first period before the transmitted magnetic field turns “on” again, and mostly relatively medium frequency resistive components (and low-frequency resistive components) are manifest during periods between the above periods within the first period.        2) U.S. Pat. No. 4,942,360 describes a multi-frequency system for which the transmit signal may be any form, but as an example, sine-waves are suggested.        3) U.S. Pat. No. 5,537,041, WO 2005/047932 describes a multi-period rectangular-wave transmitting metal detector.        4) Australian application 2006903737 describes a pulse-induction-like transmission but with a finite constant transmitted magnetic field during the receiving period. In terms of receive frequency resistive components, this is similar in response to the pulse induction system described above.        
Whilst all of the above describe means to cancel soils with resistive components independent of frequency, they do not describe means to cancel higher order effects for soils with very small but significant variations in resistive components not being quite independent of frequency, including the resistive frequency spectrum being slightly dependent on the magnitude of the transmitted magnetic field. This said soil resistive component frequency dependence limits the detection depth in some high magnetic permeable soils in the art described above.
The inventor has contrived a means to address this problem and thus increase detection depth in such soils.
Other objects and advantages of the present invention will become apparent from the following description, taken in connection with the accompanying drawings, wherein, by way of illustration and example, an embodiment of the present invention is disclosed.